Reflow oven and methods of treating surfaces of the reflow oven

ABSTRACT

A reflow oven used to join electronic components to a substrate includes a chamber housing having surfaces that are in contact with heated air mixed with contaminants, including flux, and an intermediate layer selectively applied to the surfaces of the chamber housing. The reflow oven may include fabricating the intermediate layer with a foam material, including foaming polymers, e.g., epoxy, polyurethane, polyester, and silicone, or a non-foam material, including non-foaming polymers, e.g., polytetrafluoroethylene and polyimide. A method of treating surfaces of a reflow oven exposed to contaminants, including flux, is further disclosed.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

This application relates generally to the surface mount of electroniccomponents onto a printed circuit board by employing a reflow process,and more particularly to a reflow soldering oven that is designed toheat the printed circuit board during the reflow process.

2. Discussion of Related Art

In the fabrication of printed circuit boards, electronic components areoften surface mounted to a bare board by a process known as “reflowsoldering.” In a typical reflow soldering process, a pattern of solderpaste is deposited onto the circuit board, and the leads of one or moreelectronic component are inserted into the deposited solder paste. Thecircuit board is then passed through an oven where the solder paste isreflowed (i.e., heated to a melt or reflow temperature) in the heatedzones and then cooled in a cooling zone to electrically and mechanicallyconnect the leads of the electronic component to the circuit board. Theterm “circuit board” or “printed circuit board,” as used herein,includes any type of substrate assembly of electronic components,including, for example, wafer substrates.

As stated above, present day reflow ovens have heating and coolingchambers. To achieve a consistent reflow process profile, flux needs tobe extracted and collected away from the heating/cooling chambers. Toachieve flux removal, there are two types of reflow ovens—air reflowovens and inert atmosphere reflow ovens. With the air reflow ovens, fluxis extracted by an exhaust system. With the inert atmosphere reflowovens, a flux management system is used to extract flux from theheating/cooling chambers.

Both flux removal systems suffer from well-known shortcomings. With bothsystems, flux continues to deposit onto the inner walls of theheating/cooling chambers. Over time, the flux collected on the chamberwalls create problems during production as excess flux may drip backonto actual production printed circuit boards, thereby potentiallycontaminating or otherwise compromising the attachment of componentsonto the printed circuit boards.

With current production requirements, it is desired to continuouslyoperate fabrication equipment, including reflow ovens. Thus, whencontemplating scheduled maintenance of the fabrication equipment, it isfurther desirable to keep down-time as short as possible. During suchscheduled maintenance, the removal of flux on chamber walls is generallynot addressed. Thus, ongoing flux contamination exposed to the printedcircuit boards being produced would remain. Over time, excess flux mayalso cause premature failures of components of the reflow oven,including blowers designed to facilitate air circulation within thereflow oven chamber.

SUMMARY OF THE DISCLOSURE

An aspect of the disclosure is directed to a reflow oven of the typeused to join electronic components to a substrate. In one embodiment,the reflow oven comprises a chamber housing including surfaces that arein contact with heated air mixed with contaminants, including flux, andan intermediate layer selectively applied to the surfaces of the chamberhousing. Embodiments of the reflow oven may include fabricating theintermediate layer with a foam material. In one embodiment, the foammaterial includes foaming polymers, e.g., epoxy, polyurethane,polyester, and silicone. In another embodiment, the intermediate layerincludes a non-foam material including non-foaming polymers, e.g.,polytetrafluoroethylene and polyimide. The intermediate layer mayinclude a film or foil applied to the surfaces of the chamber housingwith an adhesive.

Another aspect of the disclosure is directed to a method of treatingsurfaces of a reflow oven exposed to contaminants, including flux. Inone embodiment, the method comprises selectively applying anintermediate layer on the surfaces of the reflow oven. In someembodiments, the intermediate layer is applied by spraying, or byapplying a light pressure when adhering a film or a foil with adhesive.Embodiments of the method may further include operating the reflow ovensuch that contaminants, including flux, adhere to the intermediatelayer, and removing the intermediate layer from the surfaces of thereflow oven. In one embodiment, removing the intermediate layer from thesurfaces of the reflow oven includes peeling the intermediate layer fromthe surfaces. In a particular embodiment, the peeling may be achieved byusing a device to peel the intermediate layer from the surfaces. Thedevice may include a putty knife. The method of removing theintermediate layer from the surfaces of the reflow oven may includeapplying a chemical to the intermediate layer and wiping off or peelingthe intermediate layer from the surfaces.

The method further may comprise, after removing the intermediate layer,applying a new intermediate layer on the surfaces of the reflow oven.Applying the intermediate layer on the surfaces of the reflow ovenincludes spraying or brushing the intermediate layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1 is a schematic view of a reflow soldering oven of an embodimentof the disclosure;

FIG. 2 is a perspective view of a reflow oven chamber of the reflowsoldering oven;

FIG. 3 is a schematic cross-sectional view of an interior of the reflowoven chamber; and

FIG. 4 is an enlarged cross-sectional view of a wall of the reflow ovenchamber.

DETAILED DESCRIPTION OF THE DISCLOSURE

For the purposes of illustration only, and not to limit the generality,the present disclosure will now be described in detail with reference tothe accompanying figures. This disclosure is not limited in itsapplication to the details of construction and the arrangement ofcomponents set forth in the following description or illustrated in thedrawings. The principles set forth in this disclosure are capable ofother embodiments and of being practiced or carried out in various ways.Also the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” “having,” “containing,” “involving,” andvariations thereof herein, is meant to encompass the items listedthereafter and equivalents thereof as well as additional items.

Solder paste is routinely used in the assembly of printed circuitboards, where the solder paste is used to join electronic components tothe circuit board. Solder paste includes solder for joint formation andflux for preparing metal surfaces for solder attachment. The solderpaste may be deposited onto the metal surfaces (e.g., electronic pads)provided on the circuit board by using any number of applicationmethods. In one example, a stencil printer may employ a squeegee toforce the solder paste through a metallic stencil laid over an exposedcircuit board surface. In another example, a dispenser may dispensesolder paste material onto specific areas of the circuit board. Leads ofan electronic component are aligned with and impressed into the solderdeposits to form the assembly. In reflow soldering processes, the solderis then heated to a temperature sufficient to melt the solder and cooledto permanently couple the electronic component, both electrically andmechanically, to the circuit board. The solder typically includes analloy having a melting temperature lower than that of the metal surfacesto be joined. The temperature also must be sufficiently low so as to notcause damage to the electronic component. In certain embodiments, thesolder may be a tin-lead alloy. However, solders employing lead-freematerials may also be used.

In the solder, the flux typically includes a vehicle, solvent,activators and other additives. The vehicle is a solid or nonvolatileliquid that coats the surface to be soldered and can include rosin,resins, glycols, polyglycols, polyglycol surfactants, and glycerine. Thesolvent, which evaporates during the pre-heat and soldering process,serves to dissolve the vehicle activators, and other additives. Examplesof typical solvents include alcohols, glycols, glycol esters and/orglycol ethers and water. The activator enhances the removal of metaloxide from the surfaces to be soldered. Common activators include aminehydrochorides, dicarboxylic acids, such as adipic or succinic acid, andorganic acids, such as citric, malic or abietic acid. Other fluxadditives can include surfactants, viscosity modifiers and additives forproviding low slump or good tack characteristics for holding thecomponents in place before reflow.

One embodiment of a reflow soldering apparatus for soldering the circuitboard assembly is shown in FIG. 1. Such apparatus are sometimes referredto as reflow ovens or reflow soldering ovens in the art of printedcircuit board fabrication and assembly. The reflow soldering oven,generally indicated at 10 in FIG. 1, includes a reflow oven chamber 12in the form of a thermally insulated tunnel defining a passage forpre-heating, reflowing and then cooling solder on a circuit boardpassing therethrough. The reflow oven chamber 12 extends across aplurality of heating zones, including, in one example, three pre-heatzones 14, 16, 18 followed by three soak zones 20, 22, 24, each zonecomprising top and bottom heaters 26, 28, respectively. The soak zones20, 22, 24 are followed by four spike zones 30, 32, 34, 36, for example,which likewise include heaters 26, 28. And finally, three cooling zones38, 40, 42 follow the spike zones 30, 32, 34, 36.

A circuit board assembly 44, including deposited solder paste andelectronic components, is passed (e.g., left-to-right in FIG. 1) througheach zone of the thermally insulated reflow oven chamber 12 on afixed-speed conveyor, indicated by dashed lines at 46 in FIG. 1, therebyenabling controlled and gradual pre-heat, reflow and post-reflow coolingof the circuit board assembly. In the preliminary pre-heat zones 14, 16,18, the board is heated from ambient temperature up to the fluxactivation temperature, which may range between about 130° C. and about150° C. for lead-based solders and higher for lead-free solders. In thesoak zones 20, 22, 24, variations in temperature across the circuitboard assembly are stabilized and time is provided for the activatedflux to clean the component leads, electronic pads and solder powderbefore reflow. Additionally, VOCs in the flux are vaporized. Thetemperature in the soak zones 20, 22, 24 is typically about 140° C. toabout 160° C. for lead-based solders and higher for lead-free solders.In certain embodiments, the circuit board assembly may spend aboutthirty to about forty-five seconds passing through the soak zones 20,22, 24.

In the spike zones 30, 32, 34, 36, the temperature quickly increases toa temperature above the melting point of the solder to reflow thesolder. The melting point for eutectic or near-eutectic tin-lead solderis about 183° C., with the reflow spike being typically set about 25° C.to about 50° C. above the melting point to overcome a pasty range ofmolten solder. For lead-based solders, a typical maximum temperature inthe spike zones is in the range of about 200° C. to about 220° C.Temperatures above about 225° C. may cause baking of the flux, damage tothe components and/or sacrifice joint integrity. Temperatures belowabout 200 ° C. may prevent the joints from fully reflowing. In oneembodiment, the circuit board assembly is typically maintained at atemperature within the spike zones 30, 32, 34, 36 above the reflowtemperature for about one minute.

Next, in the cooling zones 38, 40, 42, the temperature drops below thereflow temperature, and the circuit board assembly is cooledsufficiently to solidify the joints and thereby preserve joint integritybefore the circuit board assembly leaves the reflow oven chamber 12.

A flux extraction/filtration system (not shown) may be provided toremove contaminant materials from the gas generated by the reflowsoldering oven 10. In one embodiment, an input gas duct may be connectedto or between selected zones to provide fluid communication from thereflow oven chamber 12 to the flux extraction/filtration system. Anoutput gas duct may be connected to or between the selected zones toprovide fluid communication from the flux extraction/filtration systemback to the reflow oven chamber 12. In operation, a vapor stream iswithdrawn from the reflow oven chamber 12 through the input gas duct,through the system, then through the output gas duct and back to thereflow oven chamber. Similar constructions of input gas ducts, systemsand output gas ducts may be likewise positioned to withdraw vaporstreams from or between other zones of the reflow soldering oven 10.

Turning now to FIG. 2, the several zones (e.g., pre-heat zones 14, 16,18, soak zones 20, 22, 24, and/or spike zones 30, 32, 34, 36) includingheaters 26, 28 of the reflow oven include a reflow oven chamberassembly, which is generally indicated at 50. In the shown embodiment,the reflow oven chamber assembly 50 may embody one or more zones. Itshould be noted that the reflow oven chamber assembly 50 may beconfigured to have any suitable number of zones needed or requiredwithin the reflow soldering oven. Also, it should be noted that FIG. 2illustrates the upper reflow oven chamber assembly 50. A similar lowerreflow oven chamber assembly may be provided in addition to or in lieuof the upper reflow oven chamber assembly to deliver heated air frombelow the printed circuit board as the board travels through the reflowoven.

Referring to FIGS. 2 and 3, the reflow oven chamber assembly 50 includesa rectangular-shaped chamber housing generally indicated at 52 having atop 54, two relatively longer sides 56, 58, two relatively shorter ends60, 62, and a bottom, which functions as a diffuser plate 64. In oneembodiment, the chamber housing 52 is fabricated from stainless steel.An air blower device 66 is provided on the top 54 of the chamber housing52 to direct air from an inlet 68 provided in the top 54 of the chamberhousing 52 to the reflow oven chamber 12. Air is exhausted out ofplenums 70, 72 provided along respective sides 56, 58 of the chamberhousing 52 toward an outlet 74, which is also provided in the top 54 ofthe chamber housing 52. The chamber housing 52 is configured to encloseand mount the components of the reflow oven chamber assembly 50, and isfurther configured to be suitably secured within the reflow oven chamber12 of the reflow soldering oven 10.

The diffuser plate 64 distributes air from the reflow oven chamberassembly 50 to the reflow oven chamber 12. In a certain embodiment, thediffuser plate 64 consists of 192 holes in a staggered pattern toprovide consistent, uniform airflow to the printed circuit board 44.These holes are stamped from sheet metal material such that they form aconverging nozzle that results in a uniform airstream. The arrangementis such that airflow through the reflow oven chamber assembly 50generated by the air blower device 66 with air entering the inlet 68 andexiting the diffuser plate 64. Specifically, air enters the inlet 68 asillustrated by arrows A and exits through the outlet 74 as illustratedby arrows B through the plenums 70, 72.

The surfaces of the chamber housing 52, including the diffuser plate 64and the plenums 70, 72, may be treated with an intermediate layer ofremovable material to enable the easy removal of flux and othercontaminants that build up over time on these surfaces. Specifically,the intermediate layer is applied by any suitable method during amaintenance procedure to the clean the stainless steel surfaces of thechamber housing 52. As flux builds up over time, the coating ofintermediate layer is removed along with the built-up flux andassociated contaminants. Thus, the easy removal of the intermediatelayer and flux reduces the flux clean-up process significantly andeliminates flux contamination during production. Following the removalof the intermediate layer and flux contaminants, a new intermediatelayer is applied or coated on the chamber walls, including the diffuserplate 64, during the maintenance procedure.

FIG. 4 illustrates a chamber wall 78 having a surface 80 with anintermediate layer or coating 82 applied to the surface of the chamberwall. As shown, over time, flux 84 builds up on the intermediate layer82, and requires removal for the reasons described above. One manner inwhich the intermediate layer 82 is removed from the surface 80 of thechamber wall 78 is by using a device 86 to peel the intermediate layerfrom the chamber wall. The intermediate layer 82, while adhering to thesurface 80 of the chamber wall 78, is sufficiently removable to beeasily peeled away from the chamber wall with the device 86.

It should be understood that the provision of an intermediate layer 82may be applied to reflow ovens having air or inert atmospheres and toreflow ovens having flux management systems. Thus, use of the term “air”herein is meant to include any gas within the reflow oven chamber,whether inert or not inert.

In certain embodiments, the intermediate layer exhibits the followingproperties:

good adhesion to stainless steel; capable of withstanding a generaloperating temperature within the reflow oven chamber 12 of 400° C., withhigh oven temperatures nearing 600° C.; easily removable from stainlesssteel; and easily disposable. For example, in one embodiment, theintermediate layer material is a foam material, selected from foamingpolymers including epoxy, polyurethane, polyester, and silicone. Inanother embodiment, the intermediate layer material is a non-foammaterial, selected from non-foaming polymers includingpolytetrafluoroethylene (“PTFE”) and polyimide. In yet another example,the intermediate layer may be a non-foam material that can betransformed into a foam material by adding a chemical or foaming agent.

Methods of applying the intermediate layer include, and are not limitedto, spraying and brushing the material onto the surfaces requiringcoating. For example, any suitable spraying method may be employed,e.g., by a pressurized can. The spraying or brushing may be achievedeither manually or automatically.

During a maintenance operation, in addition to other maintenanceprocedures, surfaces of the chamber housing may be accessible forcleaning. In one embodiment, the intermediate coating enables thesurfaces to be peeled away from the surfaces of the chamber housing byhand or by using any suitable instrument, e.g., a device similar to aputty knife. The intermediate layer is designed to attract fluxcontaminants, while enabling the easy release of contaminant-exposedcoating. Other methods may be used to remove the contaminatedintermediate layer, such as wiping, brushing, and the like. Stillfurther other methods may be used to remove the contaminatedintermediate layer, such as an application of one or more chemicals ontothe intermediate layer to break down the intermediate layer.

Having thus described several aspects of at least one embodiment of thisdisclosure, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the spirit and scope ofthe disclosure. Accordingly, the foregoing description and drawings areby way of example only.

For example, other intermediate layer materials may be selected forapplication on surfaces of the reflow oven chamber. For example, theintermediate layer material may be a polyphenol, an oil/lubricant,films, liners, performs, foils (e.g., aluminum foil), porous coatings,inorganic “friable” coatings, and removable panels (e.g., glass andstainless steel panels) and/or sheets (plastic or metal) that areadhered to the surfaces of the reflow oven chamber with adhesive, whichis removable from the surfaces by peeling, wiping or any other suitableremoval method.

What is claimed is:
 1. A reflow oven used to join electronic components to a substrate, the reflow oven comprising: a chamber housing including surfaces that are in contact with heated air mixed with contaminants, including flux; and an intermediate layer selectively applied to the surfaces of the chamber housing, the intermediate layer including a foam material, selected from foaming polymers including epoxy, polyurethane and polyester.
 2. A reflow oven used to join electronic components to a substrate, the reflow oven comprising: a chamber housing including surfaces that are in contact with heated air mixed with contaminants, including flux; and an intermediate layer selectively applied to the surfaces of the chamber housing, the intermediate layer including a non-foam material, selected from non-foaming polymers including polytetrafluoroethylene and polyimide.
 3. The reflow oven of claim 2, wherein the intermediate layer can be transformed into a foam material by adding a chemical or foaming agent. 